Fibre-reinforced polymers, such as long-glass fibre-reinforced polypropylene (LGF-PP), are known for their use in structural parts such as in various parts of the structural frames of vehicles. Such compositions are available in a number of variants, differing for example in fibre length or the in the type of polymer used for embedding the fibres. These materials are usually made available as granules or pellets, being created from suitably prepared strands of fibres.
On the high end, much effort is made on uniform distribution of fibres across pellets or granules. This poses some technological challenges in the manufacturing process, which usually at least consists of a step of embedding the fibres of a multifilament strand in a molten polymer composition. The fibres may for example be immersed in a polypropylene melt, cooled down after immersion, and cut into pellets or granules. Prior to and during immersion, the pulling force or load on the strand may be lowered or released in order to enable the individual fibre filaments of the strand to spread such as to yield a dispersed fibre distribution within the pellets.
A disadvantage of this method is that it is difficult to achieve proper distribution. The additional steps for enhancing fibre dispersion can only properly be performed at low speed for various reasons, e.g. since the individual filaments are more prone to breaking than the strand as a whole. The manufacturing process for making such high-end good quality LGF-PP pellets is therefore rather slow, limiting the yield of the process significantly and thereby resulting in higher costs.
Another method of manufacturing long-glass fibre-reinforced polypropylene (LGF-PP) is disclosed in International application no. WO 2009080281. In this method, use is made of an impregnating agent that is applied to a multifilament strand comprising a plurality of continuous fibre filaments (or fibres) wherein the strand(s) are surrounded by a thermoplastic polymer material by means of a sheathing step. The sheathed embedded strand is then cut into pellets. The impregnating agent is used to enhance dispersion of the fibres within the thermoplastic polymer matrix during downstream processing such as injection moulding, making any additional steps for enhancing dispersion during the step of making the pellets superfluous. As a result, the manufacturing process may be performed at much higher speeds, while still providing good quality pellets. The costs of making the LGF-PP granules or pellets are therefore much lower.
Applying the impregnating agent to the multifilament strand, however, still forms a challenge. The impregnating agent consists of a low viscous liquid at application temperature, for example a highly branched polyethylene wax mixed with a hydrocarbon oil or wax like a paraffin oil. The viscosity level is low, as a low viscosity ensures proper penetration of the strand to reach the inner filaments.
A known manner of applying a liquid coating to a single-filament fibre, such as an optical fibre, is to use a die through which the fibre filament is conveyed. The coating agent is applied by feeding the agent to a pressure chamber through which the fibre filament is conveyed, thereby immersing the fibre filament in the pressurized coating agent. The application of the pressure chamber requires the diameter of the entrance and exit openings of the coating device to be controlled accurately. A too large gap between the optical fibre and the entrance or exit die may result in coating coming out of the coating device.
DE102010045428 discloses a method for preparing a composite material consisting of a fibre strand that has been impregnated with a matrix material. The composite material is prepared by pneumatically conveying a fiber strand through a transport channel and adding matrix material to the fibre strand while it is so transported. The matrix material is added through an impregnation channel. The method and device of this publication are directed to the preparation of a composite material that is ready for (end) use to manufacture articles. Typically matrix resins, such as for example polypropylene, are high molecular weight materials. The present invention is directed to the application of a relatively low molecular weight impregnating agent to a multifilament strand followed by applying a sheath over the impregnated multifilament strand.
WO2006119752 discloses a method of applying a functional additive or a portion of a polymeric matrix material to a fibrous material wherein the fibrous material is conveyed through a blowing channel, together with the functional additive and or the portion of the polymeric matrix material under a pressure above ambient pressure and at a speed of more than 20 ms.
A problem with multifilament strands used for producing fibre reinforced polymer compositions, is that the shape and diameter of the strands varies across the length and width thereof. The term multifilament strand herein should be interpreted broadly as meaning a plurality of bundled fibre filaments, wherein a fibre filament refers to a single individual fibre. The term strand therefore also includes yarns (which are collections of filaments or strands twisted together) and rovings or roves (a collection of strands wound together). With respect to the varying shape and diameter, it is to be understood that the strands are usually provided to the manufacturing method as wound on bobbins, where the ends of strands of multiple bobbins are lashed together such as to form a single roving wound on multiple interconnected bobbins. At the lashes the thickness of the multifilament strand is approximately doubled. The lashes for example cannot be made by attaching the ends of the strands in each others prolongation, i.e. end-to-end (which would keep the diameter increase within limits), but instead the strands are usually linked by aligning them side-by-side and sewing them together using a lashing line. In addition, the multifilament strand usually is not circular in cross section but generally is rectangular in shape. In view of the unwinding of the bobbins the multifilament strand usually twists before entering the device for application of the impregnation agent.
As a result of the diameter variations in the strands, the device for applying the impregnating agent or coating agent should have entrance and exit openings, and a transport section, having a diameter in cross section being at least slightly larger than the largest expected local diameter of the strands. Otherwise, the strand may become stuck in the die and break, causing delay and loss in the manufacturing process. It is thus not possible to use the abovementioned method used for single filament fibres for applying a coating agent or an impregnating agent to the fibre filaments of a multifilament strand. As will be appreciated, methods and arrangements for coating or surface processing of (single-filament) optical fibres are usually dimensioned such as to accurately fit the cross-sectional shape of such a fibre. In order to achieve the desired optical performance, the cross-sectional shape and diameter of an optical fibre is usually well defined and consistent across its length. Therefore, there is no need to account for variations in solutions for coating an optical fibre.
In order to properly provide also the inner filaments of a multi-filament strand with impregnating agent, a much higher pressure is required. In case the entrance and exit openings are made large enough to convey also the thicker parts of a multifilament strand (such as the lashes), a sufficiently large pressure in the transport section of the die cannot be achieved for applying the low viscous coating liquid (or impregnating agent), since this would cause the coating liquid to flow out of the die openings.